Run arbitrary package scripts

This runs an arbitrary command from a package's "scripts" object.
If no package name is provided, it will search for a package.json
in the current folder and use its "scripts" object. If no "command"
is provided, it will list the available top level scripts. The env command
can be used to list environment variables that will be available to the script
at runtime. If an "env" command is defined in your package it will have
precedence instead.

run[-script] is used by the test, start, restart, and stop commands, but can
be called directly, as well.

As of npm@2.0.0, you can
use custom arguments when executing scripts. The special option -- is used by
getopt to delimit the end of the options. npm will pass
all the arguments after the -- directly to your script:

npm run test -- --grep="pattern"

The arguments will only be passed to the script specified after npm run
and not to any pre or post script.